After that (or at same time ) seems like gears are the best bang for the buck. I haven't yet as I will likely supercharge this early next year and stock 3.15s may be perfect fit.
2011 Black Mustang GT Premium
Corsa XO & Sport AB/Airaid/Bama 93R
Best 1/4 mile to date 60' 2.009
12.538 ET @112.33
100+ Deg 85 Hum RPR 3PM 1/2 tank street tires